This invention relates generally to high power lasers and, more particularly, to arrays of fiber amplifiers configured to produce a powerful composite beam. To obtain a desirably high total beam power, radiation emitting fibers are conventionally arranged in a two-dimensional array, sometimes referred to as a side-by-side array (SBSA), in which the phases of the emitters are controlled to render them mutually coherent. The beams combine in the far field as they diverge and diffract, forming the composite beam. The performance of such arrays is, however, significantly limited by the necessarily low “fill factor” of the array. Even if the fibers, which are cylindrical in shape, are closely packed in a hexagonal pattern, the fill factor is reduced by the spaces between adjacent fibers. One measure of the optical performance of such an array is the Strehl ratio of the composite beam, defined as the ratio of the on-axis intensity of the beam to the on-axis intensity that would have been obtained with a diffraction limited optical system with the same aperture and total power at the same range. A Strehl ratio of unity or 100% is indicative of an ideal beam, but this cannot be achieved in an SBSA, especially a fiber array, because the fill factor is generally significantly lower than 100%. For a closely packed array of cylindrical fibers, the fill factor as calculated by simple geometry is π/(2√{square root over (3)}), which is approximately 90.7%. If one also takes into account that the radiation from each fiber end has a centrally peaked, near Gaussian, profile the effective fill factor is even further reduced, to approximately 74%.
For the typical two-dimensional array of emitting fibers and associated lenses, the resulting far field light distribution pattern has reduced on-axis power and a significant fraction of the emitted power is radiated as sidelobes that do not contribute to the available beam power. Another difficulty with the closely packed fiber array is that heat generated in the array cannot be easily conducted away to its perimeter, which leads to temperature gradients that can adversely affect alignment of the individual beams.
An alternative to coherent combination of fiber outputs is to combine the multiple beams incoherently, i.e., without regard to their relative phases. Spectral beam combining (SBC) uses a diffractive grating to combine multiple beams of different wavelengths. The grating functions like a prism in reverse, combining the beams of different wavelengths along a single output axis. Although the spectrally combined output has no unwanted sidelobes, it has a relatively wide spectral width and not all the spectral components may be equally transmissible through the atmosphere. Moreover, each source must have a fairly narrow spectral width if the technique is to be scaled up to a large number of input beams, and each source must have long-term frequency stability. For these and other reasons, SBC does not have good prospects as a high power laser source.
Although it is possible to improve the effective fill factor of a fiber array to some degree by the use of refractive optics, ideally it would be desirable to eliminate the effect that fill factor has on the Strehl ratio of a composite beam. The present invention provides a way to accomplish this. It would also be desirable to eliminate the thermal control problems associated with closely packed arrays of fibers and associated lenses. The present invention also achieves this goal.